posix_mkfifo

Description

posix_mkfifo() creates a special
FIFO file which exists in the file system and acts as
a bidirectional communication endpoint for processes.

Parameters

pathname

Path to the FIFO file.

mode

The second parameter mode has to be given in
octal notation (e.g. 0644). The permission of the newly created
FIFO also depends on the setting of the current
umask(). The permissions of the created file are
(mode & ~umask).

Return Values

Returns TRUE on success or FALSE on failure.

Notes

Note: When safe mode is enabled, PHP checks whether
the files or directories being operated upon have the same UID (owner) as the
script that is being executed.

User Contributed Notes 7 notes

For non-blocking, fopen'd read access to a "half-connected" pipe (created with /usr/bin/mkfifo, posix_mkfifo, etc.), I just go ahead and do:<?php $fh=fopen($fifo, "r+"); // ensures at least one writer (us) so will be non-blockingstream_set_blocking($fh, false); // prevent fread / fwrite blocking?>

The "r+" allows fopen to return immediately regardless of external writer channel. You then have to use your own conventions to track $fh as a pseudo-read-only resource, since fwrite would technically be permitted as well. I've successfully used this approach on Linux with PHP 4.3.10 and PHP 5.2.4 with both half-connected (no writer yet) and pre-connected (writer already waiting) pipes, polling with stream_select as usual.

Here is a possible solution to what - tech at kwur dot com- mentioned:

I faced the problem where i had a process (a server) that needed to take care of socket connection, and in the meanwhile get some data from the database. I didn't wanted to make the clients wait for the query execution time, so i decided to make a separate process that executes the query on the DB, and the two would communicate over a pipe. Of course i didn't wanted the server blocking if no data was available. So what i come up with is to use stream_select() , and to overcome the mentioned problem, i would fork the process, open up the pipe for writing in the child, this way the parent won't block when it opens the pipe.

This is still not a solution: if I listen to commands on a pipe and output status on a separate pipe, PHP will block on both opens because something else has not already connected to this pipe. Because I can't do a low-level fcntl() to to set O_NONBLOCK or something like it, this always locks up and is really stupid. The only way I can get it to work is to spawn seperate subshells with system() and have them cat, or echo respectively and then the pipes work properly...usually? Its alot of trouble that we can't set the blocking on the open!!

A way to have a non-blocking pipe reader is to check first if the pipe exists. If so, then read from the pipe, otherwise do other stuff. This will work assuming that the writer creates the pipe, writes on it, and after that deletes the pipe.

"On some systems (but not Linux), pipes are bidirectional: data can be transmitted in both directions between the pipe ends. According to POSIX.1-2001, pipes only need to be unidirectional. Portable applications should avoid reliance on bidirectional pipe semantics."

Linux pipes are NOT bidirectional.

Also, it appears to me that the use of fifo (named) pipes in php is pretty pointless as there appears to be NO way of determining whether opening (let alone reading) from it will block. stream_select SHOULD be able to accomplish this, unfortunatly you cannot get to this point because even trying to OPEN a pipe for read will block until there is a writer.

I even tried to use popen("cat $name_of_pipe", 'r'), and even it blocked until it was opened for write by another process.